Known examples of such a relief valve device include a device described in Patent Literature 1 (Japanese Unexamined Patent Publication No. S63-318372). In the known relief valve device, two relief valves are disposed in parallel to each other in a valve case. Each of the relief valves is structured as follows.
A relief valve chamber is provided in the valve case. An Inlet hole of the relief valve chamber is communicatively connected to a hydraulic pump and to a hydraulic oil chamber of and a hydraulic device. An outlet hole of the relief valve chamber is communicatively connected to an oil tank. A valve seat member having a valve seat is pressed onto a right end wall of a left space of the relief valve chamber. A valve member having a valve surface is inserted into a right space of the relief valve chamber so as to be movable in a left-right direction, and the valve member is biased toward the valve seat of the valve seat member by a relief spring. When high-pressure oil is supplied to the inlet hole of the relief valve chamber and the hydraulic pressure in an inlet-side portion of the relief valve chamber, which is close to the inlet hole, exceeds a set pressure, the valve surface of the valve member is separated from the valve seat of the valve seat member against the biasing force of the relief spring, with the result that the relief valve is opened.